The increased use of optical components in communication and data processing systems has created an increased need for a method for measuring optical inhomegenities in optical components. For example, in fiber optic based communication systems there is a need to measure splice losses and the location of non-reflecting fiber breaks. Similarly, there is a need for a methodology for characterizing optical components such as silica based optical planar wave guides and LiNbO.sub.3 wave guides.
One method for analyzing inhomegenities in optical fibers is optical time domain reflectometry. In this method, a light pulse is transmitted down the optical fiber and the Rayleigh backscattered light resulting from the interaction of the light pulse with an inhomogeneity in the fiber is measured. The time delay between the incident light pulse and reflected light provides information on the location of the inhomogeneity. The amplitude of the backscattered light signal provides information on the degree of inhomogeneity. In conventional pulsed techniques, the measurement of the backscattered light becomes more difficult as spatial resolution is improved. Higher spatial resolution simultaneously results in lower levels of backscattered light power and increased noise power due to larger receiver bandwidths.
White light interferometry or optical low-coherence reflectometry provides a technique that allows several orders of magnitude improvement in both sensitivity and spatial resolution compared to conventional time domain methods. Spatial resolutions of 20 to 40 microns have been reported using this technique. This resolution is equivalent to the resolution that would be obtained using pulse widths of a few hundred femtoseconds with conventional pulse techniques. For these resolutions, the average backscattered levels for standard telecommunications fibers are of the order of -115 dB. Reflection sensitivities have been limited to values close to the backscattered levels due to the noise intensity of low-coherence optical sources. However, a reflection sensitivity of -136 dB has been demonstrated at a wavelength of 1.3 microns using a high-power superluminescent diode and a balanced detection scheme to minimize the effects of noise [Takada, et al., "Rayleigh Backscattering Measurements of Single-Mode Fibers by Low Coherence Optical Time-Domain Reflectometry With 14 mm Spatial Resolution", Appl. Phys. Lett., 59, p. 143, 1991].
While the low-coherence reflectometry technique taught by Takada, et al. provides the resolution and sensitivity to perform the measurements in question. the apparatus is significantly more complex than a conventional Michelson interferometer. The apparatus achieves its increased signal to noise ratio by using a balanced detector to subtract one component of the noise. To construct this balanced detector, additional optical and electrical components must be added to the system which increase the system cost and complexity. In addition, the range of distances over which measurements can be made is reduced by a factor of two relative to a Michelson interferometer. Hence, the technique taught by Takada, et al. has a smaller scan distance than a conventional Michelson interferometer.
Broadly, it is an object of the present invention to provide an improved low-coherence reflectometry measurement apparatus and method.
It is a further object of the present invention to provide a low-coherence reflectometry system with signal-to-noise performance comparable to that achieved by Takada, et al., in an apparatus having the complexity of a Michelson interferometer.
It is yet another object of the present invention to provide a low-coherence reflectometry system with improved signal-to-noise that preserves the scan distance of a conventional Michelson interferometer.
These and other objects of the present invention will-become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.